Abrasive articles and method of making same



H. R. HOUCHINS 2,730,439

Jan. 10, 1956 Filed March 19, 1953 IN V EN TOR.

6 ATTORNEY Y HENRY 1?. HOUCH/NS.

Jan. 10, 1956 Filed March 19, 1955 EFFICIENCY H. R. HOUCHINS 2,730,439

ABRASIVE ARTICLES AND METHOD OF MAKING SAME 2 Sheets-Sheet 2 I0 I I I II no I30 I50 I60 I-IARDNESS FIG. 5

INVENTOR.

HENRY R. HOUCH/NS United States Patent ARTICLES AND METHOD OF MAKINGSANIE Henry R. Houchins, Niagara Falls, N. Y., assignor to TheCarborundum Company, Niagara Falls, N. Y., a corporation of DelawareApplication March 19, 1953, Serial No. 343,296 7 Claims. (Cl. 51-307)ABRASIVE posed for use in the making of ceramic bonded abrasiveproducts. For example, certain borosilicate compositions have been usedin bonded silicon carbide abrasive products, as disclosed in Lombard eta1. U. S. Patent No. 2,332,241 and Lombard U. S. Patent No. 2,343,218,but those borosilicate compositions which have been developed for thebonding of granular silicon carbide and other oxidizable abrasive grainshave been found none too satisfactory for the making of fused aluminabodies. That is only to be expected since it has been common experience.over the years that that whichmay constitute an excellent material forthe bonding of one kind of abrasive particle is not necessarily of valuein the bonding of other kinds of abrasive particles towards which itoften exhibits little or no bonding power or on the other hand is foundto be so reactive as to attack and destroy or otherwise interfere withthe abrading ability of the material. Therefore, it is not surprisingthat those particular borosilicate compositionsfound so useful inbonding silicon carbide have not been found particularly acceptable forbonding granular fused alumina. Also, other borosilicate compositions,such as the lead borosilicate glass bonds disclosed in Harvey U. S.Patent No. 2,316,161, proposed for use in the bonding of otherabrasivematerials, including granular alumina have been deficient orunsatisfactory in' one respect or another and therefore not been whollyacceptable. 1

Consequently, this invention should not be construed as covering allborosilicate compositions nor should it be interpreted as covering thebonding of all types of abrasive materials. On the contrary it isconcerned with the bonding of granular fused alumina by means ofspecific types of alkali borosilicate compositions which have been foundto be. especially well suited for use with fused alumina particles withmany attendant advantages.

It is desirable in making vitreous bonded, fused alumina abrasiveproducts, firstly, that the bond be capable of wetting and adheringtothe particles of alumina with tenacityiu order thatithe strength of thebonded article will be j sufiicient to resist breakage of the articleunder grinding conditions, and which strength will permit operation atgrinding wheel speedsat which grinding or cutting action will be mosteffective and efiicient. It is also desirable that thoroughbondvitrification will take place without requiring excessively highvtemperatures or extended periods of firingfor reasons of economy ofmanufacture. A low firing temperature is further preferred in order thatmetallic vreinforcing means and other metallic elements such as mountingspindles or shafts incorporated in the 2,730,439 Patented Jan. 10, 1956ice abrasive body at the time of molding will not be damaged during thesubsequent firing of the article.

Furthermore, it is important and desirable that the bond have anaffinity or adhesiveness toward the alumina particles which will promotethe requisite high strength in the fired article with a minimum amountof bond. It is also desirable that the vitrified bond will be of asufficient hardness to allow the manufacture of abrasive wheels andother articles which will have the high hardness grades required forgrinding many substances for which a softer grade of ceramic bond iswholly unsuited. The vitreous bonding compositions heretofore acceptedas being most satisfactory for the making of fused alumina abrasiveproducts have been found wanting in one or more of the aforementionedproperties as a bond.

It is an object of the present invention to provide vitreous bonded,fused alumina articles embodying borosilicate vitrified bondingcompositions which minimize or avoid many, if not all, the disadvantagesor undesirable features of prior art bonds.

It is a further object to provide vitrified bonded, fused aluminaarticles having improved properties and cutting characteristics.

Inaccordance with the present invention I have discovered that vitrifiedbonded, fused alumina abrasive articles such as grinding wheels havingimproved properties and performance can be made by including lithia invitrified alkaline oxide borosilicate bonding compositions as at least apart of the alkali metal oxide contained inthe compositions. The fusedalumina particles constitute-the abrasive element of the articles andare therefore the predominant ingredient of the abrasive article,amounting to'at least 50% by weight of the article and usually a muchhigher proportion of the article. The lithia appears to impart to thebonding composition an unexpect edly high afiinity for the aluminaparticles so that the latter are thoroughly wet by the bond at lowfiring temperatures in the neighborhood of 1000 C. and even lower.Furthermore, the herein-described vitrified bonding compositionscontaining lithia retain this high afiinity and/or bonding action towardgranular fused alumina even when the bonding compositions contain highpercentages of silica in the range of 60% or more, for example 69%.Consequently, I have found it possible to make vitreous-bonded aluminaarticles with a hard and exceedingly vitreous bond containing a minimumof crystalline matter even when the articles are fired at temperaturesof 1000 C. and lower. I have further found that when the aluminaparticles are bonded'by vitreous borosilicate compositions containinglithia in accordance with the present invention the fluidity of thevitreous bonding compositions at the firing temperature and theirwetting of and/or adherence to the alumina abrasive particles permitsthe use of a lesser amount of the bond than has heretofore beenconsidered necessary for the provision of the requisite high strength,or in the event that the maximum amount of strength is desired, providesan increase in strength Without increasing the amount of bond. Moreover,the permissible presence of large amounts of silica imparts a hardnessto the bond which has heretofore been lacking in vitreous bonds fired attemperatures comparable to the temperatures at which the present bondsare fired. The bonding compositions herein employed usually contain lessthan 10% of alkali oxides which alkali oxides include from about 1.5% to5% of lithia. The bonding compositions also contain up up 15% boricoxide and over 60% of silica. Although the borosilicate bondingcompositions made following the teachings of the present inventionusually contain 60% or more of silica in order to promote and attain abond of high hardness, it is permissible to use lower amounts of silicawherever bond hardness is not a prime factor and extremely low firingtimes and temperatures are a leading consideration and a softer bond ispermissible, or even desirable. The lithia, in these softer bondingcompositions will still effect its wetting action upon the aluminaparticles and lower the amount of bond required below that required werethe lithia omitted. Ithas been found desirable to include a smallpercentage of alumina in the bonding compositions together with smallamounts of alkaline earth or other fluxes.

In order that the invention may be more clearly understood, reference ismade to the drawing in which Figure l is a diametrical cross-sectionalview through an abrasive wheel made in accordance with the presentinvention, and in which a metallic reinforcing ring has been embedded;

Figure 2 is a perspective view of a mounted abrasive wheel made inaccordance with the present invention;

Figure .3 is a view at high magnification depicting schematically thestructure of a fragmentary portion of a bonded alumina abrasive bodymade in accordance with thepresent invention and showing the dispositionof the vitreousbondand the presence of intergranular voids throughoutthe article;

Figure 4 is a similar highly magnified view of an alumina abrasive bodyin which a typical prior art vitreous bond is used; and

Figure 5 is a graph of efliciency vs. wheel hardness comparing theabrasive Wheels made in accordance with the present invention andembodying a lithia-containing borosilicate bond with abrasive wheels inwhich the bond is a vitrified borosilicate composition containing nolithia.

The following examples, which are presented for illustrative purposesonly and are not intended to limit the invention in any way, show morespecifically the manner in which vitreous bonded, fused alumina abrasivearti- A raw batch of the above composition is fused together to form awell-fused homogeneous fluid mass which is quenched by pouring it intowater, and the water-quenched ginaterial dried and ground to a finenessof 200 mesh and Vitrified bonded, fused alumina internal grinding wheels4" in diameter and 1" in thickness with /2" arbor holes are'made asfollows: A molding composition suitable for pressure molding is preparedfrom a mixture of the abovedescribed pulverized vitreous frit, fusedalumina abrasive particles and dextrine in the following proportions:

Parts by weight 60 gritsize fused alumina abrasive grain 91 Pulverized,lithia-containing borosilicate frit (as above) 9 Dextrine 1 /2 Water 2/2 The above molding composition is pressed at 1500 pounds per squareinch to form a wheel of the desired size and shape and the moldedarticle removed from the mold, dried, and fired at a temperature of 1000C. over a total finng period of ten hours. A suitable firing schedule isone requiring four hours for bringing the articles up to a temperatureof 1000 C., holding at that temperature for two hours, followed by fourhours to bring the article down to room temperature or slightly above.The fired articles are removed from the kiln and dressed by conventionalprocedure.

The vitrified bond of the finished article by calculated oxide analysisis found to have the following percentage composition:

SiOz 69.10 A1203 2.91 Coo .33 MgO .63 K20 2 00 NaO 3 B203 12 96 LiOz3.35 F2 4.57 FezOz .20

EXAMPLE II Reinforced grinding wheels such as that shown in Figure 1wherein the bonded abrasive wheel body 5 having an arbor hole 6 isreinforced by an annular steel ring 7 have been made from thecomposition set forth in Example I and following the same operationalprocedure given under Example I with the exception that in filling themold an annular steel reinforcing ring 7 is embedded within the moldedshape during the filling operation by placing the ring in the mold afterthe mold has been partially filled with the molding mixture and thenplacing the remainder of the molding mixture in the mold on top of thering. If desired, that part of the molding mixture initially placed inthe mold before the ring is placed in the mold can be subjected to lowpressure before inserting the ring so that the ring will be assured ofproper positioning. Obviously, one or more additional reinforcing ringscan be similarly embedded in the wheel during the molding operation, orthe reinforcing elements may be of another shape, such as steel rods,bars, screening or the like. The article is then finally pressed so thatthe ring or other element takes the desired position, such as that shownin the drawing. The low firing temperature required for full maturingand vitrification of the bond is advantageous in that it is notdetrimental to the reinforcing ring or rings or other reinforcingelements which retain all their original strength and thereforeaccomplish their purpose of strengthening the abrasive body.

EXAMPLE III The same bonding composition and abrasive molding mixtureused in Examples I and II can also be used to make mounted abrasivewheels such as that illustrated in Figure 2 in which the bonded abrasivebody 9 is molded about a stainless steel spindle or shaft 10 which hasone end embedded within the abrasive body 9 during the moldingoperation. This is most readily accomplished by providing the bottomplunger of the mold'with a central opening of the same diameter asthe.stainless steel shaft so that the end of the shaft extends upwardlyinto the mold barrel to the extent to which it is desired to embed theshaft within the body. The firing temperature required for maturing thebond is insuflicient to damage the metal shaft. The above-describedmethod of making mounted abrasive articles wherein the mounting spindleor shaft is embedded in the abrasive body at the time that it is moldedand prior to the firing of the article is of obvious advantage over-thepractice heretofore required with vitreous bonds maturing at highertemperatures where it has been necessary to fire the abrasive bodyseparately and then cement the desired shaft or spindle within a holeprovided in the fired abrasive shape. The difliculties involved in thelatter method of centrally locating the spindle with the accuracynecessary for true grinding is apparent. The elimination of suchdifficulties, as well as the greater ease of such one step molding ofthe complete mounted'arthe article. same amount of bond is used as wasused in making the bonded abrasive structure of Figure 3, the strengthof the body is substantially less than that of the body shown in Figure3.

'bond is present is undoubtedly due to the greater effecticle, furtheradds to the advantages already derived from the use of the present bond.

Figure 3 of the drawing represents schematically the structure ofvitrified bonded, fused alumina abrasive bodies bonded by thelithia-containing borosilicate bonding compositions of the presentinvention. As therein shown the fused alumina abrasive grains 11 areheld together by a vitreous bond 12 which, during the firing of thearticle, has flowed along the surfaces of the abrasive particles tothoroughly wet them and bond them together by small fillets 13 of thebond-located in the small angular spaces between or formed by thecontacting points or edges of the abrasive particles. The disposition ofthe bond in the manner described leaves the larger intergranular porespaces 14 of the article substantially open and unfilled with bond. As aresult the abrasive body has-a fast and cool, free cutting action.

By way of comparison, Figure 4 depicts a fused alumina abrasive body inhighly magnified form in which the alumina particles 15 are bonded bythe same amount of a vitrified borosilicate bond 16 which in compositionis representative of common prior art practice and does not containlithia. It is to be noted from the structure of the article shown inFigure 4 that the bond has not wet the surfaces of the abrasiveparticles efiectively nor shown any pronounced affinity toward theabrasive particles. Consequently, instead of flowing along the surfacesof the abrasive particles and collecting in the angular spaces betweenthe contacting corners and edges of the adjoining abrasive grains, ithas to a large extent remained where it was lodged during molding,clogging 'up the intergranular pores or spaces of the abrasive bodywhere it is of little effect in promoting the strength of As a result,in spite of the fact that the The illusion that Figure 3 gives that lesstiveness with which the 'bond has been distributed and uniformly coatedover the entire surfaces of the abrasive particles in the structure inFigure 3. Furthermore, reverting to Figure 4, the bond contains asubstantial amount of fine crystalline material 17 so that it is not infully vitrified condition. As alreadystated, the intergranular pores 18have been so filled with the bonding composition that freeness ofcutting action and coolness of out are lost to a great extent. Vitrifiedbonded, fused alumina abrasive articles made in accordance with theteachings of the present invention'wherein thebond consists of aborosilicate composition containing lithia and containing a highpercentage of silica have been shown to have a longer wheel life,resulting in more pieces being ground per wheel,

averaging 20% to 60%, and even as high as 100% above the wheel lifeshown by competitive abrasive wheels of comparative grading butembodying vitreous bonds of other composition. This greater wheel lifeis due to the added mechanical strength of the bonding composition whichis believed to be obtained because of the greater affinity of the bondfor the fused alumina surfaces, which in'turn requires the use of aminimum amount of bond to secure the necessary strength of structure.The sharp abrasive grains as a result are held securely in a minimumamount of bond and permitted to break down slowly rather than being tornfrom the bond before they have served their useful life. The provisionof a greater amount of actual pore space without sacrifice in strengthof Wheel structure has also provided a vitrified bonded, abrasive bodywhich for the first time is free cutting and which requires a minimumamount of dressing down throughout its service life. The improved wheelstructure secured through practice of the present invention has resultedin the maintenance of closer tolerances and the production of improvedsurface finishes on work surfaced with the abrasive wheels. It has alsobeen possible to provide an abrasive wheel which has a structure soversatile that one wheel can be used to perform satisfactorily for anumber of different grinding operations, thereby reducing the number ofabrasive wheels which must be kept on hand for grinding purposes.

The graph of Figure 5 demonstrates the superior cutting ability of thewheels of the present invention as compared to that of a typical type ofprior art vitreous bonded abrasive wheels. The efiiciency, as plottedalong the vertical axis, is the weight of metal removed per unit weightof wheel loss during grinding a given metal. The hardness of the wheelis plotted along the horizontal axis. It is measured by using a machinegrader, which drops a weighted tool, having a fixed contact surface, 'afixed number of times through a fixed distance onto the flat surface ofthe grinding wheel. The depth of penetration of the tool into the wheelis the measure of the wheel hardness. Thus when a hardness of 60 isreported on the graph this means that during the hardness test abovedescribed the weighted tool penetrated 6O thousandths of an inch intothe wheel. Wheels in the range of hardness covered by this graph areused for such operations as surface grinding of hardened steels. Curve Aof Figure 5 represents a series of tests run on a prior art type ofalumina abrasive wheels bonded with a borosilicate bond containing over60% silica, less than 15% boric oxide,

and no lithia, the hardness of these wheels being varied by varying theamount of bond in each wheel of this series. Curve B of Figure 5represents a similar series of tests run on abrasive wheels made inaccordance with the present invention. These wheels were bonded withvarying amounts of a bond prepared according to the teachings of thepresent invention and containing over 60% silica, less than 15% boricoxide, and about 3.5% lithia. As can be seen from Figure 5, at allhardnesses within this tested range the efficiency of the presentinvention wheels was greater than that of the prior art borosilicateglass bonded wheels. This graph, showing the results of actual grindingtests, clearly indicates the surprising grinding results which areobtained when using abrasive wheels containing over 50% granular aluminaabrasive material bonded with a lithia-containing borosilicate glassbond made in accordance with the present invention. This greaterefficiency of the wheels of the present invention means that more metalcan be removed by these wheels before they are expended, giving agreater wheel life measured in number of units ground per wheel. To showthat abrasive wheels made in accordance with the present invention havesurprisingly longer wheel lives than abrasive wheels formerly used byindustry as the best available for the same operations, the followingexamples of tests made during actual commercial production grinding withthe wheels of the present invention in comparison with prior artabrasive wheels are given.

EXAMPLE IV To determine the wheel life of wheels made in accordance withthe present invention in comparison with the wheels being used byindustry as the best available for internal grinding, tests were madeduring actual production at an industrial plant grinding the innerdiameter of inner and outer bearings rings on three Heald centerlessinternal sizematic grinders and two Heald centerless internal gagematicgrinders. Prior to the time of the test, prior art vitrified bondedaluminum oxide abrasive wheels had been used as the best available forthese jobs. During these tests the grinders were run by the operator,first using the old type of abrasive wheels so as to determine the wheellives, in number of units that could be ground with each wheel, of thesewheels for these operations. After the wheel lives of the formerly usedabrasive wheels were determined, wheels made in accordance with thepresent invention were installed on these grinders and; their-wheellives determined. The results of these tests are givensin Table Ibelow:

In each case the present invention wheels outperformed by a substantialpercentage the glass bonded alumina abrasive wheels formerly used forthis operation. This increased wheel life seemed in part to be caused bymore even and less breakdown of the present invention wheels, makingit'possible to dress these wheels less and still remain within thefinishtolerances.

EXAMPLE V To'further compare the life of the wheels of the presentinvention with the lives of the wheels currently used byindustry as thebest available, another test similar to that described in Example IV wasmade. This test was made during production grinding of the innerdiameter of inner bearing rings on a Bryant #2209-D grinder. Up untilthe'time of this test 80 grit size alumina abrasive prior art wheelsbonded with a borosilicate glass containing over 60% SiOz, less thanB203, and no lithia were being used as the best wheels available forthis operation. These prior art wheels had a wheel life for thisoperation of 200pieces per wheel. In place of these formerly usedWheels, 80 grit size alumina wheels made in accordance with'the presentinvention were tested for this operation. It was found that these newwheels have a wheel life of 300 pieces, 100 pieces, or 50%, more thanthe wheel life of the borosilicate glass bonded alumina abrasive wheelsformerly used as the best available for this operation, clearlyindicating the surprising superiority of the wheels of the presentinvention.

EXAMPLE VI To compare the life of the wheels of the present inentionwith the prior art wheels commonly used by industry for portableinternal grinding, a test was run during production at amotor plant,comparing the lives of these two types of wheels when used with an M=Bair grinder for portable grinding of forged steel crankshaft oil holes.Prior to the time of this test prior art alumina wheels were used'for'this operation. An average of 18 oil holes were being finished'with eachof these prior art wheels. Using lithia-containing borosilicate bondedwheels made according to the present invention and containing over 50%granular alumina abrasive material, an average of 40 oil holes werefinished with each wheel, an increase of 22 holes per wheel over theformerly used wheels.

The results of these tests show a marked superiority in wheel life ofthe present invention wheels when compared with the abrasive wheels usedby industry as the best available. While these tests were all made oninternal grinding operations, it is believed that similar results areobtainable for other types of grinding. Therefore, it is to beunderstood that the present invention is not to be limited to internalgrinding Wheels, but is intended to cover abrasive Wheels for othertypes of grinding.

Having described the invention it is desired to claim:

1. A bonded, fused alumina article comprising at least 50% fused aluminagrains and a vitrified borosilicate bond containing by oxide analysisapproximately 69% silica, 13% boric oxide, and under 10% alkali oxides,the alkali oxides including approximately 3% of lithium oxide.

2. A bonded, fused alumina article comprising fused alumina grains and avitrified borosilicate bond containing by oxide analysis approximately69% silica, 13% boric oxide, and under 10% alkali oxides, the alkalioxides including approximately 3% of lithium oxide.

3. A bonded, fused alumina article comprising fused alumina grains and avitrified borosilicate bond containing by oxide analysis approximately69% silica, 13% boric oxide, 3% alumina, and under 10% alkali oxides,the alkali oxides including approximately 3% of lithium oxide.

4. A vitrified bonded, fused alumina article comprising fused aluminagrains and a vitrified, lithia-containing borosilicate bond containingat least silica, up to 15% boric oxide and from 1.5 to 5% lithia.

5. A vitrified bonded, fused alumina abrasive article comprising anabrasive body mounted upon a metal shaft, said abrasive body comprisingfused alumina abrasive grains and a vitrified, lithia-containingborosilicate bond containing at least 60% silica, up to 15% boric oxideand 1.5 to 5% lithia.

6. An abrasive article in accordance with claim 4 in which the abrasivebody contains a metal reinforcing element embedded within the body ofsaid article.

7. A vitrified bonded, fused alumina article comprising at least 50%fused alumina grains and a vitrified, lithiacontaining borosilicate bondconsisting essentially of at least 60% silica, up to 15% boric oxide,from 1.5-5% lithia, and up to about 5% combined fluorine.

References Cited in the file of this patent UNITED STATES PATENTS2,316,161 Harvey Apr. 13, 1943 2,332,241 Milligan et a1. Oct. 19, 19432,334,266 Houchins Nov. 16,1943 2,343,218 Lombard Feb. 29,1944 2,478,626Gregoriefi Aug. 9, 1949

3. A BONDED, FUSED ALUMINA ARTICLE COMPRISING FUSED ALUMINA GRAINS AND AVITRIFIED BOROSILICATE BOND CONTAINING BY OXIDE ANALYSIS APPROXIMATELY69% SILICA, 13% BORIC OXIDE, 3% ALUMINA, AND UNDER 10% ALKALI OXIDES,THE ALKALI OXIDES INCLUDING APPROXIMATELY 3% OF LITHIUM OXIDE.